Technical Field
The present invention generally relates to lithographic materials for extreme ultraviolet (EUV) and electron beam (E-beam) lithography, and more particularly to inorganic hardmask processing used in EUV lithography.
Description of the Related Art
Semiconductor fabrication typically involves transfer of a pattern from a mask to a resist using lithography, and transfer of the pattern from the resist to a hardmask through etching. The pattern can then be transferred from the hardmask to a semiconductor material through further etching processes. In general, photolithography (in contrast to e-beam lithography, for example) uses light to form an image of the mask on a photoresist material, where the incident light can cause a photo reaction. Light for photolithography has progressed from wavelengths in the range of 436 nm (blue light) to 365 nm (near ultraviolet (UV)) to 248 nm (deep UV) to a wavelength of 193 nm. The wavelength of light has moved to smaller and smaller wavelengths in part because the smallest feature size that can be printed is determined in part by the wavelength, λ, of the light used. Another factor that can affect the smallest printed feature size is the numerical aperture, NA, of the projection optics. The depth-of-focus (DOF) is also determined by λ and the numerical aperture NA, which is also typically a factor in resolving small features. The DOF can relate to a visible change in the image related to exposure dose, e width, sidewall angle, and resist loss. As feature sizes decrease, their sensitivity to focus errors increases.
In extreme ultraviolet lithography (EUVL) the extreme ultraviolet light (which also may be referred to as soft x-ray) has wavelengths from 124 nm down to 10 nm, and in particular for intended semiconductor processing, about 13.5 nm, as generated by a laser-pulsed tin (Sn) plasma source. The 13.5 nm EUV light is currently the focus of the next generation of photolithography tools and processes.
Electron-beam lithography (often abbreviated as e-beam lithography) is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing). One advantage of electron-beam lithography is that in some examples it can draw custom patterns (direct-write) with sub-10 nm resolution.
High resolution patterning using Extreme Ultraviolet (EUV) lithography is typically carried out with a combination of dark field masks and EUV resists utilizing positive tone development (PTD). Among the different families of positive tone EUV resists, chemically amplified resists is a common material platform, using aqueous tetramethylammonium hydroxide (TMAH) in the PTD step. In some examples, for the subsequent image transfer of the EUV patterned structures into the underlying stack using reactive ion etch (RIE), a silicon-based hardmask layer directly located under the EUV resist is used in combination with selective etch processing, This hardmask can be a spin-on hybrid material, e.g. silicon-containing organic layer, or a vacuum-deposited inorganic layer such as polycrystalline silicon (p-Si) or amorphous silicon (α-Si). In some instances, high silicon content is associated with high etch selectivity. Therefore, the vacuum-deposited inorganic layers, such as polycrystalline silicon (p-Si) or amorphous silicon (α-Si) are often employed.
However, these materials are characterized by the presence of a native surface oxide layer (SiOx) about 1 nm thick. The EUV resist structures feature poor adhesion to the silicon layer due to the acidic nature of the silanol (SiOH) termination of the SiOx surface layer. Typically, a surface priming process that replaces the hydrophilic SiO—H with a hydrophobic SiO—Si (CH3)3 termination is carried out utilizing vapor-applied hexamethyldisilazane (HMDS), which can improved adhesion of the EUV resist pattern to the silicon layer underneath.
However, the standard HMDS vapor priming process is insufficient at improving the adhesion of high resolution EUV patterns, such as tightly-pitched sub-20 nm. EUV resist lines. Therefore, a method is needed to improve the adhesion of EUV photoresists to the silicon hardmask.